The potential for using a flywheel as an energy storage medium is well known. From a materials technology standpoint, the primary factor that controls the energy storage efficiency of a flywheel system is the strength to density ratio of the material used for the flywheel. Conventionally, flywheels have been made of metals such as high strength steel. Steel, however, may not be a suitable material for a flywheel that must store large amounts of energy efficiently. Specific disadvantages of metallic flywheels include the high weight of the flywheel and the potential for dangerous fractures associated with rotating the flywheel at high speeds. In addition, conventional metals are not well suited for high energy flywheels because their high density results in excessive loading during operation at high rim velocities.
Due to the disadvantages of metallic flywheels, wound fiber reinforced resin matrix composite flywheels have been developed. Such flywheels, which have circumferentially-oriented fibers, have the potential for achieving high strength to density ratios in the direction of fiber reinforcement because of the availability of high strength, low density fibers. Although resin matrix composites can achieve high hoop strength (strength parallel to the fiber direction), the mechanical properties of the composites are very anisotropic. For example, unidirectional resin matrix composites may possess hoop strengths between about 1000 MPa to about 2500 MPa, while the radial strength (strength perpendicular to the fiber direction) of these composites may be only between about 10 MPa to about 20 MPa. This anisotropic behavior limits the use of resin matrix composites for flywheels. In many potential flywheel designs, particularly high speed flywheels, the flywheel's radial strength is exceeded prior to achieving full utilization of the inherent high hoop strength of the flywheel. As a result fractures occur, making the flywheel unsuitable for many high speed applications.
Therefore, there exists a need for an energy storage flywheel device capable of achieving improved radial strength while maintaining sufficient hoop strength for operation at high rim velocities (e.g., 600 meters per second or greater).